This invention relates to image-guided surgery, and in particular, to a method and system for generating subsurface 2-D shadowgraph views of a patient.
In a typical image-guided surgical procedure, pre-operative scan data, e.g., CT or MRI scan data, of a patient region is obtained. This is done conventionally by immobilizing a patient during a scan with a CT or MRI imaging machine. The patient is then moved to a surgical setting for treatment of the target region. During the surgical procedure, the scan data is used to reconstruct hidden or subsurface images of the patient target region, to guide or assist the surgeon in the surgical procedure. For example, the scan data may be used to reconstruct a subsurface image of the target region as seen from the position and orientation of a surgical instrument being held by the surgeon.
It is important, of course, for the coordinates of the scan-data images being shown to the surgeon to closely match the coordinates of the actual target region. For this reason, it is important to calibrate the scan data with the actual position of the patient, keeping in mind that the scan data is obtained in one location, and the patient is moved to another location for the surgical procedure. Where the target region is a patient""s head region, this coordinate matching may be done by placing fixed-position fiducials on the patient""s head during the scanning procedure, and retaining the fiducials at the same head positions for the surgical operation. Coordinate matching can then be done by aligning the positions of the fiducials in both reference frames. This approach relies on the fact that the fiducials placed on rigid, non-deformable structure (e.g., skull) are retained in substantially the same position (patient coordinates) for both the scanning and surgical procedures.
For other surgical regions, e.g., the spine, it is virtually impossible to retain fiducials at fixed patient coordinates between the scanning and surgical operations. In this case, it is necessary to recalibrate the positions of the fiducials in the patient coordinate system every time the patient moves or is moved. Ideally, the surgeon would want to forego the use of fiducials altogether, since their placement involves additional inconvenience to the patient and surgeon.
It would therefore be useful to provide an improved system for registering pre-op scan data in a patient coordinate system during a surgical operation, to be able to use the scan data to accurately construct subsurface image data corresponding to the actual patient coordinate system.
Also during a surgical operation, a surgeon will often wish to check the exact placement of surgical cut, or placement of a tool or implant at the patient site. This can be done conventionally, by taking a fluoroscopic image of the patient region, and displaying the 2-D image to the surgeon. Unfortunately, the surgeon will be limited in the number or times and duration of fluoroscopic imaging, due to the need to limit the amount of x-ray exposure to both the patient and medical personnel.
It would therefore be desirable to provide a method and system of virtual fluoroscopy that allows a surgeon to view a patient region xe2x80x9cfluoroscopicallyxe2x80x9d from a number of different angles, and over extended periods of view, without actually exposing the patient and others in the surgical theatre to exposure to x-rays.
In particular, it is would be useful to construct virtual fluoroscopic images in real time, e.g., without significant computation time.
The invention includes, in one aspect, a method of obtaining desired subsurface views of a patient target site. The method includes the steps of (a)obtaining volumetric scan data of a surgical site of a patient, and storing the scan data in digital form in a scan-data file, where the scan data is composed of voxels having defined coordinates and associated M-bit tissue-density values, (b) with the patient positioned in a surgical station, selecting a point in space representing a virtual fluoroscopic irradiation source, (c) defining a plurality of rays extending between the source point and each of a plurality of points in a two-dimensional XY array of points, where (i) the array points correspond to pixels in an XY pixel array in a display screen, and (ii) at least some of the rays pass through a plurality of such voxels in the patient target region, (d) for each ray, summing the M-bit values of the voxels along that ray, (e) displaying the image constructed of gray-scale values representing the summed M-bit density values at each pixel in said display screen, and (f) repeating steps (a)-(e) until a desired displayed subsurface view of the patient target site is obtained.
In a preferred embodiment, for rapid and real-time generation of images, the display screen contains a two-dimensional XY array of pixel elements, where each element contains multiple N-bit registers for receiving digital-scale values for each of multiple colors, and summing step (d) includes, for each ray, (i) distributing the M bits of the voxel density values among the multiple N-bit registers of the associated pixel, such that one or more bit positions of each M-bit value is assigned to a selected register of that pixel, and (ii) summing the M-bit values of the voxels along that ray by (iia) individually summing the one or more bit-position values in each associated pixel register, and (iib) determining the sum of the M-bit values along the associated ray from the values in the individual registers.
Where the pixel elements in the graphics screen each contain four 8-bit registers, corresponding normally to blue, green, red and alpha registers, the density values in each voxel are 8-bit numbers, and the bit positions of the 8 bit voxel-density values are distributed, for example, (a) two digits each to each of the registers, or 3 digits, 3 digits, 1 digit and 1 digit to the four registers.
The selecting step may be carried out using a hand-held pointer to indicate the position and orientation of the desired view, where the position and orientation of the pointer can be tracked by a position-tracking device. The method, for example, allows for views along the long axis of the patient, i.e., views not otherwise available to the user.
In a related aspect, the invention includes a system for use in obtaining desired subsurface views of a patient target site. The system includes (a) a scan-data file for storing, in digital form, volumetric scan data of a surgical site of a patient, where the scan data is composed of voxels having defined coordinates and associated M-bit tissue-density values, (b) a display screen containing a two-dimensional XY array of pixel elements, (c) a pointer for indicating selected positions and orientations from which subsurface views are to be reconstructed, and (d) a computational device. The latter is operatively connected to the data file, pointer, and display screen for (i) determining the position and orientation of the pointer, (ii) defining a plurality of rays extending between a selected pointer position and each of a plurality of points in a two-dimensional XY array, where (iia) the array points correspond to array pixels in the display screen, and (iib) at least some of the rays pass through a plurality of scan-date voxels in the patient target region, (iii) for each ray, for each ray, summing the M-bit values of the voxels along that ray, and (iv) displaying the image constructed of gray-scale values representing the summed M-bit density values at each pixel in said display screen.
For use in real-time evaluation of the displayed images, the display screen contains a two-dimensional XY array of pixel elements, where each element contains multiple N-bit registers for receiving digital-scale values for each of multiple colors. The computational device performs the summing operation for each ray by (i) distributing the M bits of the voxel density values among the multiple N-bit registers of the associated pixel, such that one or more bit positions of each M-bit value is assigned to a selected register of that pixel, and (ii) summing the M-bit values of the voxels along that ray by (iia) individually summing the one or more bit-position values in each associated pixel register, and (iib) determining the sum of the M-bit values along the associated ray from the values in the individual registers.
These and other objects and features of the invention will become more fully apparent when the following detailed of the invention is read in conjunction with the accompanying drawings.